1. Field of the Invention
The present invention relates to the field of mobile digital communications, and more particularly, cellular communications.
2. Prior Art
The Cellular Digital Packet Data (CDPD) network described in Cellular Digital Packet Data System Specification V1.0, Jul. 19, 1993, is designed to operate as an extension of existing data communications networks. From the mobile subscriber's perspective, the CDPD Network is simply a wireless mobile extension of traditional networks.
CDPD shares the transmission facilities of existing Advanced Mobile Phone System (AMPS) cellular telephone networks. CDPD Mobile Data Base Station (MDBS) equipment is located at a cellular carrier's cell site and can be integrated with existing AMPS base station cellular equipment. CDPD provides a non-intrusive, packet switched data service that shares frequency channels with AMPS without impacting AMPS service.
CDPD provides packetized data communications via Mobile End Systems (M-ESs, the term used by the CDPD standard to describe the communication terminals used by subscribers to the CDPD service) which are analogous to mobile telephones. The packetized nature of the data transmissions from the M-ESs allows many CDPD users to share a common channel, accessing the channel only when they have data to send, and otherwise leaving it available to other CDPD users. For users whose data transmission requirements are characterized by numerous transmissions of short to medium duration, CDPD is a far more cost effective solution than using circuit switched cellular data modems. CDPD is also superior to other existing packet data systems because it has been adapted as a national standard by the cellular industry and will be available nationwide wherever cellular voice capability exists.
By comparison to AMPS, the infrastructure requirements of CDPD are very small. The multiple access nature of the system makes it possible to provide substantial CDPD coverage to many users simultaneously with the installation of only one CDPD MDBS radio on a given sector. Unlike AMPS, which uses in-band FSK signaling to establish call connections and an out-of-band (control channel) signal to control the mobility, CDPD uses a sophisticated set of in-band protocols to control channel access and mobility and to manage the transfer of data from end to end through the CDPD network.
Although the CDPD system shares existing AMPS radio frequency channels, AMPS calls are given first priority, and they are always able to preempt the use of any channel being used by CDPD. (The cellular service provider may, however, opt to dedicate a channel or channels to CDPD usage. In this case AMPS calls will never attempt to preempt the channel(s) occupied by CDPD.) The most common configuration of the MDBS uses a technique commonly referred to as "RF sniffing" to detect the presence or absence of AMPS activity on a given channel (in some more highly integrated systems the MDBS can be given channel information directly by the AMPS equipment via a data interface). If the sniffer finds that a channel is not in use, the MDBS may establish a CDPD network on this channel by transmitting on the forward link. M-ESs will acquire the forward link, register, and then begin to transmit packets on the return channel according to a Digital Sense Multiple Access (DSMA) scheme in which the M-ESs access the channel only when they have data to send.
In particular, when an IDLE flag is transmitted by the MDBS on the forward link, it propagates to the subscriber unit and is detected by the subscriber unit, which may choose to begin transmitting. If it does begin to transmit, it begins with the transmission of a dotting sequence, consisting of a 38 bit string of alternating ones and zeroes, modulated on an RF carrier. The dotting sequence is not differentially coded, so during the transmission of the dotting sequence, the phase changes by 90 degrees (nominally) when a one is sent and by -90 degrees when a zero is sent. Because of the intersymbol interference in the GMSK (Gaussian minimum shift keyed) modulation, the actual phase change is closer to 54 degrees. The purpose of the dotting sequence is to provide a reference signal for the MDBS to lock onto to establish the bit timing in the MDBS before the end of the dotting sequence, so that bit timing is established before transmission of further data signals.
Immediately after the transmission of the dotting sequence, the subscriber unit will transmit a 22 bit synchronization signal, then data, typically 385 bits, which will include both the desired data and error detection and correction bits for the data. In an analog system, the MDBS would start receiving the RF signal containing the dotting sequence, demodulate or downshift the same and try to detect the bits therein with arbitrary timing, pulling in to an adequately accurate bit timing after some number of bits of the dotting sequence have been received, depending at least in part on how far off the arbitrary timing was to start with, signal quality, etc. Thus while bit timing would normally be established before the end of the dotting sequence, how far before the end would vary, so that detection of at least the later part of the alternating 1,0 string gives no indication as to exactly when the string will end and the data will start. The purpose of the 22 bit synchronization signal, therefore, is to provide a string of correlation bits to the MDBS, detectable by the already established bit timing, to allow the MDBS to expressly orient itself to bit positions in the string so as to be able to identify the true beginning of data in the received signal. The 22 bit synchronization signal is chosen so that it will provide a high correlation with an identical reference bit string when the two are aligned, but a poor correlation with an identical reference bit string when the two are miss-aligned one or more bits.
In a digital system, the ability to store at least some prior signals and to analyze them after the fact should frequently allow recovery of the entire dotting sequence. However the integrity of the bit alignment achievable between transmitter and receiver with the correlation with the entire 22 bit synchronization signal is still far superior to trying to align to the ends of the dotting sequence. Also the Cellular Digital Packet Data System Specification hereinbefore referred to requires an ability to detect the presence of an arbitrary or random data stream in a received signal. Thus a pure correlation to the dotting sequence, without more, would not be adequate for satisfying this specification requirement.
The purpose of the present invention is to provide methods and apparatus for the down conversion of the RF signal using a coarse/fine frequency correction, for the detection of the dotting sequence and for establishment and maintenance of receiver timing, some of the foregoing techniques also being useful in the subscriber units for forward link data communication. The present invention also allows the detection of the presence of an arbitrary or random data stream in a received signal as required by the Cellular Digital Packet Data System Specification herein before referred to.